Ablation of Jab1 results in defective axon sorting and myelination. (A and B) Semithin sections of P60 sciatic nerves from WT (A) and Jab1^−/− (B) mice. Arrows identify bundles of unsorted axons in mutant nerves. (C) Electron micrographs of P60 sciatic nerves from Jab1^−/− mice. Bundles of unsorted axons contain axons of different caliber, including those with diameter >1 µm. Bundles are entirely surrounded by Schwann cells that protrude processes within the axons; the boxed area is magnified below, and arrowheads identify Schwann cell protrusions within unsorted axons. (D) Electron micrographs of P60 sciatic nerves from Jab1^−/− mice showing sorted but not myelinated axons and discontinuous basal lamina (arrows). (E) Electron micrographs of P90 sciatic nerves from Jab1^−/− mice showing bundles of unsorted axons entirely myelinated (polyaxonal myelination). (F) Semithin and ultrathin sections of 6-mo-old sciatic nerve from WT and Jab1^−/− mice. The asterisk identifies Schwann cell degeneration, and the arrow shows axonal degeneration. Bars: (A, B, and F [left]) 20 µm; (C [top], E, and F [right]) 1 µm; (C, bottom) 1.7 µm; (D) 0.5 µm.